Objective To determine the ED50 of isoflurane and desflurane in the presence and absence of epidural lidocaine using BIS value 50 as the criterion (MACBIS50 ) .Methods Forty-eight ASA I - II patients (26 male, 22 female) aged 18-70 years were randomly divided into isoflurane group ( n = 24) and desflurane group ( n = 24).Each group was further divided into 2 subgroups: general anesthesia group(GA, n = 12) and combined general-epidural anesthesia (CGEA, n = 12) . Patients with body weight over 110kg or under 50kg were excluded. The patients were premedicated with intramuscular atropine 0.01 mg?kg-1 . In CGEA subgroup epidural anesthesia was performed at T5-6 before induction of general anesthesia. A catheter was inserted into the epidural space in a cephalad direction for 3cm. A loading dose of 10ml of 1.6% lidocaine with 1:200 000 adrenaline was given after test dose followed by 5 ml 1.6% lidocaine every 40-60 min. General anesthesia was induced with fentanyl 1 ?g?kg-1, propofol 0.5-1.0 mg?kg-1 and vecuronium 0.1-0.2 mg?kg-1 . The patients were intubated and mechanically ventilated (FiO2 = 100% , FGF = 2 L?min-1 , VT = 8-10 ml?kg-1, PET CO2 = 35 mm Hg) . Anesthesia was maintained with isoflurane or desflurane. The end-tidal concentration of isoflurane and desflurane was continuously monitored and BIS also. MACBIS50 was measured using up-and-down method. The initial concentration of anesthetic for isoflurane was 1.0% and for desflurane 5.0% . A steady state end-tidal concentration of isoflurane and desflurane was maintained for 10 min. When BIS value was 50 the anesthetic concentration for the next patient increased by the same amount. Results MACBIS50 of isoflurane in GA subgroup was 1.16% and in CGEA subgroup 0.77% . MACBIS50 of desflurane in GA subgroup was 5.18% and in CGEA subgroup 3.87% . There was linear correlation between BIS value and end-tidal concentration in each subgroup. Conclusion Epidural lidocaine significantly reduces the end-tidal concentration of isoflurane and desflurane for adequate depth of anesthesia. A linear correlation is maintained between BIS and end-tidal concentration.